Also, I know several people have created modifications to allow their animations to be controlled by physics engines. What can I add to the editor to make that easy?

I'm not planning to spend much time (at least not yet) on the runtime side of things, but it seems safe to expect support for the most common frameworks and engines. (Though I might have to ask for community help on some of them.)

Now, what features would you like? This is all totally in the planning stages, so I welcome any feedback you want to send.

I think that the XNA version is good enought for now. If you want to port the editor to other platforms, why not use MonoGame? This would reduce the time spent on the editor and focus on more important features, like the texture pivot editing, for example.